1. Technical Field
The present invention relates to a line head capable of easily realizing gray-scale display and an image forming apparatus using the same.
2. Related Art
In general, an electrophotographic toner image forming device includes a photoconductor serving as an image carrier having a photosensitive layer on an outer peripheral surface thereof, a charging unit that uniformly charges the outer peripheral surface of the photoconductor, an exposure unit that selectively exposes the outer peripheral surface uniformly charged by the charging unit so as to form an electrostatic latent image, and a developing unit that applies toner serving as a developer to the electrostatic latent image formed by the exposure unit so as to make a visible image (toner image).
A tandem-type image forming apparatus that forms a color image includes a plurality of (for example, four) toner image forming devices, which have been described above, disposed around an intermediate transfer belt. In this type of image forming apparatus, there is an intermediate transfer belt type in which toner images formed on the photoconductor by the single-color toner image forming devices are sequentially transferred onto the intermediate transfer belt and toner images corresponding to a plurality of colors (for example, yellow, cyan, magenta, and black) are superimposed on the intermediate transfer belt so as to obtain a color image on the intermediate transfer belt.
In the tandem-type image forming apparatus having the configuration described above, there is known that an LED or an organic EL element is used as a light-emitting element in a line head. In the line head having the configuration described above, exposure energy of each pixel is changed in a stepwise manner in order to improve a gray-scale level of an image that is formed. As a method of changing the exposure energy, a method of changing a lighting time, that is, a pulse width modulation (PWM) or a method of changing exposure power, that is, an intensity modulation (current modulation) has been used frequently.
As an example of gradation control, JP-A-06-079118 discloses a technique in which two pixels are arranged in the sub-scanning direction and are exposed at different timing so that an image is formed and multiple exposures are performed by superimposing pixels on a photoconductor. In the example, the gradation is displayed by performing combination of lighting of superimposed pixels. In addition, although not an example of the gradation control, an example of forming one pixel (output image) by using a plurality of sub-pixels is disclosed in JP-A-2002-292922. In a technique disclosed in JP-A-2002-292922, a pixel is divided into, for example, nine sub-pixels (3 sub-pixels in the main scanning direction×3 sub-pixels in the sub-scanning direction) for exposure. The plurality of sub-pixels are turned on at the same time regardless of positions thereof. A light source in JP-A-2002-292922 is disclosed as an ‘electroluminescent element’. However, it is considered that an organic EL material is used for the light source because the electroluminescent element is weak to humidity, for example. Moreover, in examples of using a laser beam in a light source, which are disclosed in JP-A-2002-251023, JP-A-60-154268, and JP-A-03-004244, a technique of setting the size of a spot with respect to a pixel pitch is disclosed.
However, there has been a problem that a modulation circuit for the PWM or the current modulation, which is used to perform the gradation control, is required for each pixel, and accordingly, a driving circuit of each pixel becomes complicated and large. Particularly in recent years, even though such line head is used in an electrophotographic color page printer in many cases, a high capability of displaying photo or graphic and high reproducibility thereof are requested and a high-level gradation control is needed in the case of a color image, as compared with a monochrome image. The gradation control as above is performed in a digital manner. However, in order to perform the gradation control, an amount of information, that is, the number of bits larger than the number of gray-scale levels is needed. Accordingly, the size of a gradation control circuit tends to be large, which has caused a problem of cost increase.
Further, in order to improve gradation of an image to be formed, it is difficult to reduce the spot diameter (spot diameter at the time when a light beam emitted from a light source passes through a lens array and is then imaged on a surface of an image carrier) in correspondence with the density of pixels. Even if the spot diameter can be reduced, fluctuation of the spot size or the like of each pixel becomes large due to a difference among optical characteristics, such as focusing, of pixels in a lens array. As a result, there has been a problem that uniformity of an image may be damaged.
Furthermore, in the image forming apparatus disclosed in JP-A-06-079118, there has been a problem that, since light beams output from two light-emitting parts are completely superimposed on the same position, the resolution is not improve even if the number of pixels increases. In addition, FIG. 8 of JP-A-2002-292922 shows an example where three rows of light sources are arranged in a zigzag manner. Here, nine light-emitting parts form one ‘light-emitting part group’, and projection onto a photoconductor is made in the shape unchanged. For this reason, the gradation control has not been possible. In addition, objects of the techniques disclosed in JP-A-2002-251023, JP-A-60-154268, and JP-A-03-004244 are to improve the resolution of an image. Accordingly, in the case when a pixel pitch is small, the spot size should also be small corresponding to the pixel pitch. As a result, a control operation becomes troublesome.